Mizushoubai (water business) is the term for Japan's night life activities, such as fuuzoku (similar to prostitution but none of the illegal bits), hosting and managing (entertaining at bars), and kyabakura (night clubs).
Recently, it was decided that a high school should be founded to educate the youth in these area where they'd previously had no such guidance: the School of Water Business, "Mizushou", located in Tokyo's Shinjuku ward.
The story is from the perspective of a teacher, Tanabe Keisuke, who was transferred there to help teach that school's first generation of students.
From the Old Gringo
Toritsu Mizushou first appeared as a light novel by Hikaru Morozumi. The manga series doesn't seem to share very much with the novel other than the setting, so in a way you might call it an authorized doujin.
Shinobu Inokuma's series before this was Salad Days, and you can find similar character designs there. One character in Salad Days is cloned into an entire baseball team in Mizushou. But unlike Salad Days, Mizushou is much more focused on major characters, and story arcs tend to become longer as the series goes on.
All in all, great comedy and good soap opera. Explicit sexual content has been surprisingly rare, more or less equivalent to Kenichi Sonada's Gunsmith Cats or Cannon God Exaxxion.
Inokuma has a web site, "The Garden Plot" at http://saladplanning.pro.tok2.com/main.html